*35c3 Preroll music*
Herald: It's an honor to introduce you. Sven Prüfer who is a professional in the
space business and he's going to give you a introduction to spacecraft control under
the title of space ops 101.
*Applause*
Sven: OK. Thank you very much for the kind introduction. Hello and welcome to space
ops 101. My name is Sven Prüfer, I'm a mission planning engineer at the German
Space Operations Center which is a part of the Deutsches Zentrum für Luft und
Raumfahrt and I will give you a slightly biased introduction to spacecraft control.
It's slightly biased because first of all I'm working for a particular space agency.
And secondly, because we will look at the whole thing kind of through the lens of a
mission planning engineering. Unfortunately the topic is pretty, well..
large so we won't be able to talk about everything. In particular, we will not
talk about launches, launches are pretty amazing. I'd love to see one in real life
but we can't really go into that much detail because that's a very specific and
particular topic. Also we will not talk much about human spaceflight and neither
about entry descent landing. So for example, landing on another planet. Of
course the combination of human spaceflight and landing on another planet
would be very cool to see. But I cant just talk about it right now. Okay. So instead
we will deal with one of the main segments of mission operations. So in general you
distinguish three parts. There's one. The space segments. So this is everything that
actually flies up into space. Some particular satellite or spacecraft
including its payload, so whatever it is doing up there. Then there is the transfer
segment which is, well, the launching business. And then thirdly there is the
ground segment. So we will talk among mostly about the Ground segment. So this
is everything that actually takes place on earth in order to command or use the
spacecraft in space. OK. The Ground segment itself again splits into various
subsystems. So, one of them is the the the main player when you want to actually talk
to your spacecraft. Those are the ground stations. OK. So we will definitely need
to talk about those. Secondly we need to actually know where our spacecraft is and
where it's going. This is actually done or described by the flight dynamics. Thirdly
space is at the same time very cold and also very hot. So there's the power and
thermal subsystem. Then there is attitude and orbit control which are responsible for
telling the spacecraft where it should look at and for actually figuring out how
it is oriented. Next we need to actually talk to the spacecraft. This includes
interpreting, well, receiving and interpreting the data. So this is part of
the TM/TC subsystem or the data system. And last but not least that's of course
the most important subsystem. That's the mission planning which is responsible for
scheduling spacecraft activities. OK, so the talk will kind of follow the lifecycle
of a spacecraft. We will start with the launch and early operations phase which is
called LEOP for short and that we will need to talk about orbits and flight
dynamics as well as how to actually communicate with the spacecraft. After
that we will talk about how we can, well, test and validate our spacecraft very
quickly and then we will switch to the routine phase, so when we do the actual
operations for what, of whatever the spacecraft was designed to do. This
includes data analysis, telemetry and Tele- commands, so TM/TC and also the mission
planning. And then in the end we will talk about, well, the end of the mission. So
whatever we are going to do at the end when we want to dispose of the satellite.
All right. So everything starts with the launch. Well not quite. Of course before
that we have a pretty lengthy phase of preparations. I will not actually talk
about this but this might take about something like two years in advance of the
launch in order to prepare everything to make sure that everything is running
smoothly. Once the spacecraft is strapped onto the rocket it will get, well, flown
into space and there it will be separated from the launch vehicle. From this moment
on then, it's flying by itself. And we need to actually control it. However we
don't really know right now where the launch provider will put our spacecraft. It
might actually be on its final orbit. And so for example if it's a rather low orbit
or it might be a transfer orbit to its final target orbit if it's actually
further up. Once this is - during this launch there's actually a second control
center. That's the one for the spacecraft. This is actually the control room K1 in
the German Space Operations Center. And it kind of looks like you expect a control
room to look. So in particular, there are many screens on average everybody has like
four screens. There are large ones for showing an overview of what's going to
happen. And there are many small yellow signs. These yellow signs denote the
various positions of the operators and the engineers. At the back in the center there
is one position that's called the flight director. The flight director is the
person who is in charge of the operation. So whenever there's something that needs
to be confirmed, needs to be done, that needs to be decided and he is the last
operational person to actually confirm the decision. Now in principle right after the
spacecraft is separated from the rocket. This control room actually takes over.
However there are few subtleties here. In particular right after separation the
spacecraft is somewhere we kind of know approximately where it is because we
planned this beforehand but we don't know the precise position. We first have to
acquire a signal we have to find it in space and have to set up a connection in
order to understand this we need to talk a little bit about orbital mechanics. So
first of all why does a spacecraft not fall down. Well if you look at the ISS, so
the International Space Station, it flies at an altitude of about 300 to 400
kilometers where the gravitational force of the Earth is still about 90 percent of
the one at ground. This means that you really need some horizontal speed in order
to not fall down to earth. So you need to go really fast. 7.9 km/s is the speed that
you actually need in order to not fall down on the ground. So if you're a bit
higher and some orbit then you need a bit less speed actually. OK because you're
farther away from the from the earth. OK. So we need to go very fast. Good thing to
know. Secondly we need to know at which distances we will actually be flying our
spacecraft. So this is Earth obviously, in particular the following picture will
actually be to scale approximately. So one thing one possible place where you can put
your spacecraft is low earth orbit. So that's the region below about 2000 km
altitude above ground. However 2000 are pretty high so very common are
altitudes of 600 kilometres, 500km, 700km. This is a place where you mostly do
scientific experiments, in particular Earth observation. OK. So there are many many
satellites, science scientific satellites that actually try to take pictures at
various frequencies of the earth and also this is the place where you do
reconnaissance. OK. Then they're actually a bit higher altitudes for example, this
is medium Earth orbit. So the the drawn circle is actually at an altitude of 20000
kilometers. And this is mainly used for navigational satellites. So think GPS or
Galileo, the European version. And then there's another very common type of orbit
that's the geostationary orbit. This is at an altitude of about or pretty much precisely
35786 km above ground. This is chosen in such a way that the orbital period, so the
the time it takes it to fly once around the Earth, is 24 hours. This has the
advantage that the movement of the satellite actually sync up with the, are
synchronized with the rotation of the earth, meaning that's your satellite is
kind of always at the same position as when seen from Earth. This is a particular
important for TV satellites because well imagine you would have to actually move
around your TV satellite dish all the time just because the satellite is moving.
Instead you only have to fix it once and then it's pointing in the right direction.
OK. And this is also very common place for communications satellites for the same
reason because we actually want to have a fixed position in which we have to look.
OK. In order to get there for someone geostationary orbit it's possible that the
launch provider will actually put us in some kind of transfer orbit. Yeah. They
usually then don't look like circles but rather like ellipses and and that in such
a case we would need to do additional maneuvers. So we are on the red circle. We
will fly outwards but at some point we'll touch the geostationary orbit, so the black
one. But in order to not well kind of fly back to Earth we will have to accelerate.
So this is a maneuver that we have to execute somewhat at the beginning of the
mission in order to reach our geostationary orbit. OK. So the the system
that actually deals with these considerations, calculations, Procedura,
that's the flight dynamics department. So their tasks are in particular orbit
determination. There are various ways to do this for example. Very often you can
actually ask the satellite where it is because it has GPS onboard at least
if it's a LEO, so satellite in low earth orbit so it actually knows where it is or
you can do ranging which we'll talk about in a few seconds. And from this you can
calculate the orbit. Once you have the orbit you also want to know where the
satellite is going to be located in the future. So you will do orbit propagation.
Next thing, well, we have to we might have to execute some maneuver to actually stay
where we want to be or to get where we want to be. So we need to calculate which
direction we have to thrust, we have to turn on our thrusters for how long. This
is also done by flight dynamics. And the fourth point is, well we have to talk to
the satellite. So we actually need to see it, in order to do this. And flight
dynamics can actually calculate the times and the positions or the directions rather
where the satellite's going to be. And you can see all of these tasks are pretty
numerical nature its really hard core mathematics numerics. Meaning that you
actually want to use some tools that are very well battle tested so to speak and
well one of the most common programming languages for numerical caluclations is of
course Fortran. OK, so that's really a place where Fortran is still being used,
actively being used because these libraries are just working the way they're
supposed to work. So nobody really wants to switch from there because they're just
very good. OK. Now let's go back to the control room. We have talked to our Flight
Dynamics department who have told us: Well, the satellite's going to be at a
certain position, at a certain time. Or at least that's where we expect it. So the
next thing we need to do, is we need to establish a connection to the satellite.
And for this we need a ground station. The picture you see here is actually the
ground station in Weilheim. That's in Bavaria. That's sort of the main ground
station that we use. And well, it knows where to expect the satellite. So at a
certain direction it should appear, at a certain time above the horizon and then it
tries to establish a contact. This first acquisition, as it is called, is the first
contact of the spacecraft after the separation. And this is of course a
crucial moment. Now, once it has established a connection, it tries to do
various things. First of all it needs to downlink some data. So download, but it's
called downlink. This includes telemetry, so descriptions of the state of the
spacecraft because want to make sure that the spacecraft is actually still working
after the launch. And then later this also includes downlink of payload data, for
example. So think pictures, or whatever it was that the satellite was supposed to
measure or to take. And then it will also uplink some stuff. So, for example
commands because want to tell the satellite to do something. But this might
also include for example software updates. And one other thing that the ground
station can do, is ranging. Ranging means that you sent a package or a packet to the
satellite from the ground station. This travels with the speed of light. Then the
satellite will actually reply to that signal, to that packet, and then the
answer will fly. well, will go back to the ground station. And if you measure the
time, and if you know how long the satellite takes to actually react to such
a package, you can calculate the distance from the ground station. If you do this
several times then you get kind of like a radio distance profile. And from this you
can really deduce the orbit of the satellite. OK. So let's look again to
Earth. There's a ground station. It's actually located at the North Pole here.
So that's on top. And there's a satellite. The satellite is not to scale, just in
case you were wondering. And it's actually flying on an orbit which is 600 kilometers
above the ground. This is actually to scale. Now the signals of the ground
station, they actually have to pass through the atmosphere, meaning they're
attenuated quite a bit. So you have a finite range of the ground station's
signal, and this is drawn here. So the red circle is an approximate range of
the ground station and this intersects the orbit of the satellite only at a certain
time interval, or a certain interval of the orbit. In particular, we can look at
some numbers here. If you have a satellite at 600 kilometers altitude, you get a 90
minutes period, approximately 90 minutes period, around the Earth. And the portion
of the orbit that you actually see the satellite from 1 given ground station is
10 minutes long. OK. So this means, we would expect to see the satellite every 90
minutes for 10 minutes. OK? And this is when we have to do all the downlink and
uplink. Unfortunately, it's a bit more complicated because Earth actually
rotates. This map of Earth actually shows the ground track of the satellite, so
that's the projection of the satellite onto the ground. So that's the red line.
And the problem is that after 90 minutes the satellite returns to the position
where it was before. However, Earth has actually rotated by some amount, like 90
minutes divided over 24 hours. This is why the ground tracks actually don't close
up. So instead, you get these kinds of stripes. Over Europe you see WHM, that's
the Weilheim station. It has a certain range. That's the circle-like black line.
And you can see that usually you have two contacts with the satellite per rotation.
So the third pass will already be outside of the range of the ground station. OK. So
we actually have even less contacts than what I said earlier. This picture actually
shows the same situation from the top, so from North Pole. You can see that actually
they are circles. So all the distortions that you've seen on the earlier slide, was
due to the projection that was used for the map. So this is sort of what it
actually looks like if you look from above the earth. But the other one is the
typical maps that you see. OK. So now we have found our spacecraft. We want to talk
to it. So we need to actually send a signal there. Now let's think about which
kind of frequencies we might use for this communication. Well, first of all, we
noticed that there is for example water vapor in the atmosphere which absorbs
parts of the electromagnetic spectrum. So for example here, at around 23 gigahertz,
there is an absorption peak due to water, and the higher frequencies we use, the
more actually gets absorbed. This means that we kind of want to restrict our
frequency usage to actually lower frequencies, in order to get a higher
range. But then we also have, well, maybe less data rate. So in spacecrafts, you
usually use actually the lower part of the graph that's shown here. Usually, even
below what is shown at all. So this starts at 10 gigahertz, and you use even less
frequencies or lower frequencies. For example, you might use UHF, so amateur
radio at 430 megahertz, you might use L-Band, 1 to 2 gigahertz, in particular the
main carrier frequency of GPS satellites is in this range. OK? Then there is
S-Band. So that's a very typical frequency range from 2 to 4 gigahertz which is used
for the actual commanding of the satellite. So this is an important
frequency for us, or band for us. Then there's also the X-Band. So X-Band is
higher frequency, so we expect even higher data rates. And this is usually then used
for payloads. OK? So, if you have a lot of data that you want to downlink, for
example a picture that you just took from your satellite. Also, this is being used for
deep space missions. Then there is Ku- Band. Ku-Band is used for TV satellites.
And Ka-Band. So this is now slightly above the local water vapor absorption maximum.
So this is pretty cool. There, you really have high data rates. It's been used for
various applications, whenever you need to a high data rate. However, there are some
mechanical difficulties because you have a directional antenna, so this is slightly
non-trivial but it's being used more and more often. Now if you fix such a
frequency, and you talk to the satellite, you of course need to modulate some signal
on top of that. You need some protocols which do some level of error correction
etc. I will not talk about this but in principle there are very specific
standards for space that are being used in order to assure that signals that you send
or that you receive actually get received. OK! So, we can now talk to the satellite,
we have acquired a signal. So we switch back to the control room. In the control
room, we are now very happy. So we have done the first acquisition, this is
actually when people hear applause. And then afterwards, there are a few things
that are left to do. Actually, now the work starts. So for example, the satellite
was actually running on battery during the launch and afterwards. But it needs of
course some new power. So for this you need to deploy solar panels. This is done
during the LEOP. Also, you might need to deploy antennas. I showed you various
frequency bands, and usually satellites actually have several antennas and use
several bands for different tasks. So the commanding might be done on the S-Band but
the actual downlink of the payload data might be on X-Band. For this you need an
additional antenna. So this needs to be deployed. Also, this is the time when you
do all the other maneuvers in order to reach your final orbit, and you start
switching on other components of the spacecraft. This might include for
example, star trackers. Star trackers are essentially cameras that just take
pictures of the sky, so the stars, and they compare them to some onboard database
of known star positions. And this way, the camera can figure out which direction it
is looking. If you know how the star tracker is actually mounted on a
spacecraft, you can then deduce how the spacecraft is oriented. This is important
for example if you want to take a picture, then of course you need to know, where
have you been actually looking at? So you need something like a star tracker.
Another thing that you would kind of switch on, or actually spin up during a
LEOP, would be a reaction wheel. So reaction wheels are essentially gyros that
just rotate very quickly. You spin them up, and the idea is that, well, this
stabilizes the spacecraft. Because you actually want to control the rotations in
most cases. OK. So now we hope that everything was working perfectly. We
launched the spacecraft. But unfortunately, not always everything goes
perfectly. So let's maybe dig into some example. This is TV-SAT 1. Well, I
don't have a picture but that was a TV satellite from 1987. And everything worked
as we described. So we got the first acquisition. We got some telemetry from
the spacecraft. But unfortunately, the solar arrays turned out to be only
partially deployed. That's of course a problem. And we need to diagnose this, and
we need to fix it if possible. So the first thing you have to know, is that you
kind of can't really necessarily trust all the data that you get. You have to confirm
that whatever you're seeing, is actually the case. So you have to use additional
sources. For example, in the case of a solar array, you can actually check, how
much is the power output? Is it actually less than expected if it was deployed? And
it turned out: Yes, there's not enough power. And secondly, once you notice this,
you can actually send the manual deployment command again, now. So it's
possible that the automatic solar panel deployment didn't work. So we just tried
again and unfortunately, this did not work. So it still seemed undeployed. Now
you start thinking, well, what are we going to do. And you consult usually the
satellite manufacturer. The satellite manufacturer actually also sits in the
control room during the LEOP because there happen to be many questions, so you need
somebody on hand. And they suggested, or the people who operate the satellite, they
suggested various tests to figure out what was wrong with TV-SAT 1. And I want to
present just 2 of these things you can try. One is, you can orient the spacecraft
or the satellite such that it is at a 45 degree angle towards the the sunlight, and
then you start rotating it. If you do this carefully, and you measure the power
output of the solar arrays, you can actually estimate the angle that the solar
array was deployed. So they did that and they figured out, well, they're completely
not deployed. So less than 2 degrees actually. OK. So that's a problem. Then
they did various other tests, and they came up with one possible problem. And
this is that there might be the actual stirrups, sort of the black boxes in the
picture which keep the solar array attached to the satellite during the
launch, that they might still be there. In principle, they should have been kind of
fired off, or removed, and then the solar arrays should deploy. But it looked like,
they were actually still there. So one thing you can then try, is, well, you can
again rotate the satellite in such a way that the stirrups will cause a small
shadow over the solar array. This will reduce the power output again just a tiny
little bit. So you might be able to measure this. And this way confirm that
the stirrups are still there. Turns out, this was not actually really well
measurable. So this didn't work. However, they were still able to deduce it. It was
probably the stirrups that are still there. Once you have diagnosed the
problem, you want to solve it, of course. So let's see. How can we recover such a
situation? And this is sort of where you can just follow your creativity and come
up with arbitrary solutions, and see whether you can actually try them. So one
thing we can do is, we can spin up the spacecraft. If we do this very fast, we
will have a very strong centrifugal force. So maybe an acceleration of about 1 G. And
this way, we might hope that we loosen the stirrups. Another thing you can try to do:
You can use your main engine to actually accelerate the spacecraft in a pulsed way
in order to excite resonant frequencies off the stirrups. OK. So hopefully this
might actually loosen the stirrups. Another thing you can try to do is, you
can comand the spacecraft to heat up and to cool down in some ways. And this way
actually also losens the stirrups. And the last thing you can try is, you can kind of
just try to shock the whole thing. So for example, you could deploy an antenna, in
this particular case that was the main antenna which was actually stuck beneath
the solar array. So you'd try to deploy this and hope that the force actually
pushes the solar array out. Yeah. Unfortunately, none of those worked. And
this was an unsuccessful recovery of a satellite. So in particular, the main
problem was that, well, this was a TV satellite, so it really needs the antenna,
but the antenna couldn't deploy because of the stuck solar array. So in this case,
this did not work. But usually, of course, this works and people are coming up with
very creative, very interesting solutions to all kinds of problems and get things
running. All right. So once we have our spacecraft in some kind of safe state, we
kind of conclude the LEOP and we start testing the actual properties of the
spacecraft. This is called the commissioning phase, or in-orbit-testing
of the payload. So this usually takes longer than a LEOP, might take several
months, depends on what type of mission you're looking at. This is when you
actually start or switch on the payload, and when you also verify that the payload
is working as expected. OK. So in the picture, you see a geostationary
communications satellite. So its main payload are the communication arrays, or
the antennas, in particular. So, for example, you might want to actually verify
that the antennas are working properly after the launch. So during launch, they
all get shaked up and it's really pretty intense. So you want to make sure that
they are working properly afterwards. So for example, one thing you might want to
do is, point the satellite at your ground station, you measure the strength of the
signal that you receive, then you move it slightly, you measure again the strength,
and this way you kind of get a pattern of the antenna. OK. And this is a property
of this particular one, this particular antenna that you might use later. Another
thing that you do during this time is, you checkout redundant components of the
satellite. So for example, if you have an Earth observation mission, you, as I
already mentioned, you need to know where you're looking at. So you need for example
GPS or a star tracker. Now if that fails, you obviously have a large problem because
now suddenly you don't know where you were taking photos or images. So usually, there
is quite a bit of redundancy on satellites. So there are 2 GPS receivers.
And then you can actually switch between them, and during this phase, you will test
that they are working properly. OK. So let's suppose, we have done this and
everything is working as expected. Then we start with the routine phase. The routine
phase is sort of the main phase of the operation. So that's when you actually do
the science experiments, or you start offering communication services, or
whatever it is you're doing. This picture is a picture of the mission TerraSAR-,
TanDEM-X. So those are two radar satellites flying in low earth orbit. And
they can actually make 3-dimensional maps of the ground by sending a radar signal
and then receiving it. And because they're flying in close formation, so something
like a few 100 meters apart from each other, they actually get this kind of
sterographical 3-D information. And during the routine phase, a scientist would
actually order a data take or a picture of this kind somewhere, maybe online. And
then somehow the mission would actually command this, the command center would
command this data take. It gets downlinked, and then the result will
actually given to the scientists. OK. So this is the main phase of the spacecraft's
life. So where we do this payload operations. But wait. This picture is a
picture of a joint American-German mission. That's a Grace follow-on mission,
two satellites that have a microwave or a laser link between them. And they measure
the distances in order to, well, variations of the distances in order to
deduce the gravitational field of the Earth. Last year at 34c3, there was
actually a talk about the predecessor mission. Here, actually, probably in this
room. OK. So this is the time when we do our science experiments. Furthermore, we
actually monitor the spacecraft of course because we still need to know what's
happening. Is it working properly? We will, of course, continue to handle
contingencies. But hopefully there are none anymore. And we might also adapt to
new mission requirements. So for example, well, you could actually try to devise new
kinds of experiments on the flying satellite. And for that, you might need to
upload new software which is also done during this phase. Another issue is that a
spacecraft actually ages. So for example, a battery might deteriorate. So its total
capacity actually gets smaller over time. So you need to adapt to that. For example,
if there's less power available, then you can actually do fewer data takes,
something like that. And you need to monitor this and react accordingly. OK. So
how does the monitoring work? Well, that's part of the TC/TM and the Data subsystem
or system. And the idea is that the spacecraft actually measures various
properties that it has or that describe the state, all the time. So we have a time
series of binary data, and also of numerical values. So for example, here,
the plot shows the temperature of a certain part of the spacecraft over time.
But remember, we don't have this information available live. We only get
this once we actually downlink it. OK. And then we get a huge part of the data at
once. OK. So this describes the state of the spacecraft. And there can be lots of
parameters. So for example, 20.000 telemetry parameters for one spacecraft is
possible. If you measure something once every second, you do this for a few years,
20.000 parameters, this means that you have a lot of data. So obviously, you can
do a lot of data analysis, time series analysis with that. You can do anomaly
detection, telemetry prediction, or detecting errors or problems within this
data. Also, what you need to do is, you kind of need to save this to some kind of
offline database. Because lots of other subsystems actually need this data.
Because they want to know what is the state of the spacecraft. So this is an example
for telemetry view. So this is one software that we use. It's called Geckos.
And you can see here a number of telemetry pakets. So for example, there are a few
confirmations that some checksum was correct, and that some pinging was
actually received and was being worked on. OK. So it was executed. It's time stamped,
and you get some additional information. And this is sort of the the most basic
thing you can really see. Once you know the state of your spacecraft, you actually
want to command the spacecraft to do something. This is done via Telecommands.
And on the picture here, you can see some commands that have been executed and also
some that are still to be executed. So for example, in the upper part, you see a few
pings which were not actually answered by the spacecraft. But the last one was
received and was replied to. And the operator can, for example, already load a
few Telecommands on the manual command stack, prepare them and then execute them
very quickly. This is the lower part. Notice that these Telecommands are very
specific to the spacecraft because they really need to do something there. So this
is in some way provided by the satellite manufacturer, and you have to somehow
understand all the possible things you can do. In particular, you very often don't
really want to do like very atomic things, but instead, you want to achieve a certain
task. For this you bundle the Telecommands, you can add for example also
telemetry checks, so conditions on the telemetry, and you call this a Flight
Operations Procedure. So this will be sort of a bundled thing that will executed on
the spacecraft, for the purpose of achieving a specific goal. Another thing
that's important, as I've mentioned various times, you don't see the
spacecraft all the time, meaning, you cannot really command it all the time, but
instead what you do is, you send Telecommands, but you make them time-
tagged, and then they get executed, for example, when you don't see the
spacecraft. OK. And these kinds of Telecommands are called TTC. Let's look at
an example. So this might be a set of time-tagged Telecommands for a maneuver.
OK. So at the time t zero, we want to execute some maneuver. So we want to turn
on the thrusters, this time and the position and this, and the duration of the
burn, they were calculated by the flight dynamics departments of course, but one
hour before that we actually need to check, for example, that the spacecraft is
in some some fixed state, some prepared safe state. Eight seconds later we might
actually start heating up the thrusters because the fuel needs some kind of
operational temperature. Then eleven minutes before the burn starts, you will
automatically command the switch-on of some additional telemetry. So this is kind of
like, you turn on the debug mode, OK? You just tell the spacecraft to
actually tell you, to give you more data. Then, because the burn will actually
make the the spacecraft shake quite a bit, there will be lots of alarms going off. So
at some point before the burn, you will turn off these alarms to safeguards, just
because the reaction of the spacecraft is actually expected. Then you
start rotating in the right direction of course. And at some point the burn starts.
Now this should in principle stop automatically. However you might command
an additional safeguard stop command, just to make sure that in case the other one
well kind of didn't get executed, you you stop nevertheless. And
then you kind of reverse the whole procedure to return to a mode where you
can proceed with your payload operations, okay? And this would be a sequence of
time-tagged commands that are uploaded to the spacecraft during an uplink and then
executed whenever t0 was actually taking place. All right. So there is one other
thing that I want to describe and this is mission planning, since probably they the...
Yeah. One of the lesser known subsystems and this is sort of at the
point where you have to wait between automation and manual commanding. So
suppose you have a scientist that actually wants to take pictures. So he wants to
have the satellite taking some pictures of some region. So then he has to sort of ask
if the satellite can do this and has to make a reservation. This is being taken
care of by the mission planning system, which will then talk to Flight Dynamics to
see whether this is actually possible, give feedback to the scientist, and this
will also tell the operators, or the operating... well, the telecommand
operators to actually execute some command to take the data take. However, because of
all these kinds of little issues, problems that you can have all the time, you cannot
really automate everything. There is some kind of, some amount of manual commanding
that's still being needed. For example due to those contingencies. So what the
mission planning system internally does is it schedules activities and it tries to do
this in some consistent and conflict-free manner. Yeah, si imagine, for example, for
the, for a data take. You need to actually take the picture before you want to
downlink it. OK, so there those are two activities and they should actually take
place in some order. OK. From this, these kind of activities that were
requested by some scientists, they create, the system creates a timeline which is
then, well, provided to everybody who needs to know what the spacecraft is going
to do at some point. So here's one example. So that's one software we
use. So it's called PINTA and it shows on the x-axis the time, and up on the top,
you see these black-white things. Okay, so this is, these are actually eclipses. So
whenever the spacecraft is not in the sun or is in the sun, you can see this there.
And below that, there there are a few experiments planned, but one of them is
partially planned during an eclipse. But it has the condition that it must not take
place during the eclipse, so this gives a conflict. And the mission planning
system is responsible for identifying these kind of conflicts and
actually supplying that information to the scientist or the operator to be
resolved. One other thing you can see is this thing that we talked about at the
beginning. So you need to downlink the information from the experiment. So you
need some scheduled downlinks, downlink opportunities, and you can see two of them
actually as the green lines above the blue ground. So this is when, the next time
when the satellite actually sees the ground station and it can downlink the
results of the prior experiments. OK. So now we are doing kind of semi-automated
all our experiments. We gather a lot of scientific data, but at some point
everything has to end. So there's also the the end of the mission that you have to
consider. So in general, the mission time of a spacecraft might depend for example
on the mission goal. Imagine that the, that you have one specific experiment that
you want to do and this might be finished at some point in time. Also it might
depend on the orbit itself. So if you have a spacecraft in an altitude of 300 to 400
kilometers it will actually descend into the atmosphere within less than a year. If
you have a satellite at an altitude above, say, 700 kilometers it would take more
than 25 years to actually get down. If you are in a geostationary orbit you will
actually never come down. So another thing is, and this is mainly for geostationary
orbit, geostationary satellites, is that you have a finite amount of fuel. So at
some point you can't really keep your spacecraft at the position where it is, so
then you have to end the mission, of course. For geostationary satellites this
might take something like 15 years, for low earth orbit satellites a few years are
pretty common but very often you can actually extend the lifetime quite
considerably if you are very careful about your fuel consumption for example. Now
what are you going to do once it reaches the end of the mission? Well, this depends
again on the orbit. So for example, if you have a low earth orbit satellite then you
reserve some fuel, or you might reserve some fuel in order to actually take it to
a lower orbit such that it deorbits and disintegrates in the atmosphere within
something like 25 years. These 25 years, they are now, they nowadays pretty much
mandated by for example the FCC and also the ESA. So you really need to kind of
dispose of your spacecraft at most 25 years after the end of your mission. So
you can deorbit LEO satellites, but usually there is not enough fuel to
deorbit a geostationary orbit satellite. In that case, you will actually raise the
altitude by something like 500 kilometers and put them on the so-called graveyard
orbit, because that's a place where they are not disturbing anybody anymore, so you
can put them there and, well, kind of forget about them. OK. Well and then you
can look back at at your mission, you have spent quite a few years on that, yeah.
And, well, hopefully it was, everything was working correctly, you produced a lot
of scientific data, you're happy and with this I also want to end my talk. So thank
you very much. And enjoy the rest of the Congress.
*Applause*
Herald Angel: Thank you. There's about 10
to 15 minutes left for Q and A. This works pretty simple. You walk to a microphone.
You wave your hand and you may end up with the opportunity to ask a question, which
gets me to the "asking questions" bit: Q&A is for questions, not about
statements or how nice a speech was, etc. So keep it short. And the first question
goes to the Internet, to the Signal Angel who has been diligently monitoring IRC and
Twitter on the hashtag #hallC, so signal angel, do have a question?
Signal Angel: Yes, yes, hello. Yes, yes yes, (in the background: no Mic!) Hello,
hello, hello, hello. (Background: Need mic on the signal Angel!) Hello check check.
Herald: You need to, you know, use microphone. Get the microphone, I will
take the Question first to this microphone over here. (Background: okay)
Question: Hello, is this on? Nope. Microphone 2, please. it's not on?
(Background: number 5) Is it, is it on now?
Different Microphone: Okay great, test test? Ah. Would it be feasible to put like
4 satellites in geostationary orbit as communication relays, so we have uplink
all the time, and why is it not done? Sven: Yeah, so, this is feasible, and this
is actually being done. So for example the, the ISS, as far as I know, actually
does most of its communication via some relays, relay satellite in geostationary
orbit by NASA, but there also for some European alternatives, OK, so there's a
European data relay system, for example, that you can also use for this. This is
being used. However, it's always, I mean, money is always an important issue. OK so
if you're using somebody else's communication relay system then you of
course have to pay for that. So you some very often actually try to find a minimal
solution to your communication needs. Thank you.
Herald: OK. Next question goes to Microphone number two.
Microphone 2: Yes. This is the question from the Internet, which would like to
know about the security of the protocols and protection or encryption or anything
like that. Sven: OK. So I mean I can't really give
too many details about this because that's not my particular area of expertise, but
in principle, the the tele-commanding and, or the, or at least the telemetry is
usually encrypted. So there is a lot of effort put into that. However, for the
payload data, this is not always encrypted. For example, very famously
known are the weather satellites, so you can just receive the data and it is
transmitted and clear and you can just receive them. OK. Thank you.
Herald: Okay. Next question is from microphone number 1.
M1: So just one example, you told an example of a geosat that *inaudible* out
and it didn't work. Who does, who has final decision on, "Oh it's not working,
and we're going to drop this project, and maybe start anew" - who has the final
decision? And, in particular this geosat, while it was put on an orbit so long ago,
did they just leave it there? I mean, it's down and its *inaudible*. So,
one question: who gets the decision, and the other one is, did they leave it there?
Sven: OK, so, the decision making process is kind of involved. I haven't been part
of any mission yet that failed, so I kind of don't really know the details of that.
But in principle there's not just the flight director. So first, I mentioned the
flight director but that's actually the person in charge during the actual
operations. But there's also for example the project investigators, that the PI,
who's doing the scientific, who's having the, who's in charge of the scientific
process. There are other kinds of organizational people, and they decide
this together in some way. Okay so this is a non-trivial decision. And regarding the
other question, the, So I mean, they could still, for TVSat 1 they could still
control the satellite. So they were actually able, as far as I know, they, to
lower the orbit to actually have it burn up at some point. I think they even tried
to turn it on at some time later and I think it still worked. But nowadays I
think it has already burned up. So at least this mentioned somebody, I'm not
quite sure but, yeah, it was still usable. Well in that sense you could still lower
the orbit, eh, the orbit. So that's not a problem for the satellite.
Herald: Okay next question for microphone number 2.
M2: You mentioned a, you had a temperature time series on your, on your charts. I was
wondering what message do you use to find anomalies in this temperature time series?
Sven: Pardon, what's the question? M2: What method do you use to find
anomalies in that temperature series. Sven: Ah, well, so, I mean, there are
quite a few properties of the spacecraft that might actually deteriorate over time,
and there might be various indications for that. And you try to look for hints that
something is wrong, something that you're not noticing because nothing is failing
yet but you actually want to to see that, for example, some sliding average is
actually increasing over time. it's still below some some kind of alarm limit, but
it's actually getting worse. OK. So you, you try to do Time's series analysis for
that. Yeah. There are very various similar issues that you want to identify
M2: So it's a moving average, or ARIMA? Sven: So, this particular example or...?
M2: Yeah, I was wondering... Sven: Yeah, well, I'm not sure this this
particular example shows anything particular. So this seemed to work properly,
I guess. Yeah, so. M2: OK. Thanks a lot.
Herald: Questions *inaudible*. Sorry, next question from microphone number 1.
M1: Just talk about commands, sending commands. Does this command get sent and
interpreted by the server, or at least some kind of compilation? If you send up
binary or something like that, or an executable? Do you have a server-side
????? server-side? So, does the satellite do interpretation or do you send a
compiled command as software? Sven: OK. Well it's kind of like an API, I
mean, that you define, that actually gets provided by the, well, satellite
manufacturer. So you really send a binary command, so I might be, these protocols
are actually very effective. Yes, so they do just one thing, they make sure that
this is actually transmitted correctly, and then it gets executed. So this might
be, just switch one of the machines. OK. So there's just some binary thing that you
need to transmit to the satellite. There's of course some level of checking going on,
so for example there might be a command counter that needs to be correct, or some
kind of checksum, but apart from that this will be executed directly. However,
sometimes you also need to upload some kind of binary data. For example, imagine
that, for some reason, one of the things on your satellite moves a little bit then
the orientation is not correct anymore and you need to somehow fix this in your
internal calculations. For that you need to actually upload some rotation matrix,
for example, describing this small distortion. OK. So in that case you would
actually upload some binary data that gets put at the correct place on the onboard
computer. M1: OK.
Herald: OK, next question is from microphone number 4
M4: About the orbits. Is there much garbage on these orbits, and is this a
problem? Sven: Is there a what? Sorry.
M4: Is there much garbage, so old satellites or parts that get lost?
Sven: So so you're talking about space debris, so stuff that's flying around and
that might actually hit our satellite. Yes there is quite a bit. So satellites
actually have to do maneuvers to just, to to be on the safe side to not crash into
some, do not collide with some space debris. It's getting more and more, in
particular there was a destruction of a satellite a few years ago by the Chinese
so they tried to blow up their own satellite, and for example this created a
lot of additional debris. This is, however, the debris is actually flying on
the same orbit or approximately the same orbit as it was beforehand. OK. So instead
of large target, you now have many smaller ones. They are being tracked by various
space agencies. You can actually get their data online somewhere and I think they
will even write you an e-mail if your satellite happens to to be on a collision
course with something. M4: Can I ask a second question? Is there
any idea how to remove this, or...? Sven: So I'm not too knowledgeable about
this, but in principle there are people trying to do this. So the ESA actually has
various projects, has done a few conferences on the question how to deal
with space debris, but I'm not sure there's any really good and feasible
solution yet, but maybe in a few years, hopefully.
M4: Thank you. Sven: Thank you.
Herald: OK. Next question from microphone number 5.
M5: Yeah. I would like to add to their question. So she was talking about the
Keppler syndrome in the LEO, right? But you also talked about the graveyard orbit,
so, will we build a second Keppler syndrome just a little further out?
Sven: So, I'm not sure I got the last question but, so, the graveyard orbits,
they are actually for geostationary orbits, yeah? Because you can't deorbit a
satellite from there. So instead you kind of move it away from the earth.
M5: Yeah. So my question is: will we create the same problem on the
geostationary, this. Sven: Yeah, I mean, in principle this
means that there is also space debris then there, in geostationary orbit. However, I
mean, if you fixed the orbit then, well, with increasing orbit, the the, well,
there is more space left. Okay. So the density actually kind of reduces with a
larger radius. So you're not having the same problems as with LEO, yeah, so.
Because in LEO they're really, you're accumulating space debris faster than
you're actually deorbiting it. So you have to actually go through LEO to get to geo-
transfer orbits. But yeah it's not, it's not such an urgent issue there, and likely
will never be. But, who knows. M5: Thank you.
Sven: Also. And maybe also some comment. Nowadays there's kind of a shift from
geostationary orbit to actually going more LEO. Yeah. Also for communications
satellites. So this might actually maybe in long term even reduce the number of
geostationary satellites but I don't know. Herald: Okay. Next question goes to the
Internet. Signal angel: So, IRC... Hello? Yes. So,
IRC would like to know if you're concerned with SpaceX launching 5000 satellites into
low-earth orbit running at 25 000 k/h. Sven: Pardon, can you repeat that?
Signal: SpaceX is talking about launching thousands of cell satellites.
Sven: Yeah. Signal: How is that gonna work with
communications with those buzzing around. Sven: So I don't know the details about
this project but, so as far as I know, they talk about something like 4000
communications satellites in low earth orbit and as far as I remember they're
supposed to communicate via lasers. Okay, so they will actually spend sort of a
laser communication network and then you just try to route your, the information
that you have through this network. Okay. Of course this is a lot of satellites. I
don't know at which altitude they will operate, whether this will cause problems
for anybody, but as far as I know the FCC in the U.S. has already said that it's
okay to proceed with this project. So, yeah, let's see where this will lead. It's
hard to say at the moment I guess. Herald: Next question is from Microphone
number 3, and this may be the last question.
M3: I would like to know in regard to redundancy with antennas, are the
satellites built in a way that an antenna for one frequency can take over duties
for, that were actually intended for another frequency especially in two
scenarios, if the antenna receiving instructions is compromised and cannot
deploy or, for example, if the telemetry antenna is somehow incapacitated.
Sven: Right, so, on the ground, for example, an antenna might actually be able
to to serve another frequency, OK? So this is pretty common, for example in Weilheim
on one of the pictures you've seen a large antenna that can actually serve multiple
frequencies. On the satellite, I don't think this is actually done as far as I
know. However, of course, you could try to route the same kind of information through
another antenna, but it depends a little bit on the bus I guess. So for example of
the satellite bus. So on some satellites the additional antennas are actually well
kind of separate from the satellite bus, and in that case it's not feasible to
actually route the telemetry through that. But I guess in various cases this is
indeed possible, but I'm not sure, I've ever heard that it is actually being used.
Herald: OK. Thank you very much. That was the last question and this was the end of
this talk. A round of applause for our speaker.
*Applause*
Sven: Thank you.
*applause*
*35c3 postroll music*
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